1. Field of the Invention
The present invention relates to an optical disk device, and in particular, to reproduction of address information from an optical disk onto which the address information is formed in advance, such as a DVD-RAM.
2. Description of the Related Art
In an optical disk such as a DVD-RAM, a header section and a data section are formed within a sector. Address information are embossed and formed in the header section to allow random accessing.
FIG. 7 schematically shows a structure of a sector in a DVD-RAM. One sector comprises a header section and a data section. Four items of address information, ID1, ID2, ID3, and ID4 are embossed and formed in the header section. The address information of the sector can be obtained by replaying any one of the address information ID1–ID4. The address information ID1 and ID2 are formed at positions identical to each other with respect to the radial direction (first group of emboss data) and the address information ID3 and ID4 are formed at positions identical to each other with respect to the radial direction (second group of emboss data). The address information ID1 and ID2 and the address information ID3 and ID4 are formed at positions separated from each other in the radial direction. As shown in FIG. 7, when a spot 100 of laser light from an optical pickup moves to the right in FIG. 7 (when the optical disk is rotated and moved to the left), the address information ID1 and ID2 are first detected and then the address information ID3 and ID4 are detected. In a DVD-RAM, in order to improve the writing density, data is written on lands and grooves. On a land, the address information ID1 and ID2 appear at radially inward positions while the address information ID3 and ID4 appear at radially outward positions. On the other hand, on a groove, the address information ID1 and ID2 appear at radially outward positions while the address information ID3 and ID4 appear at radially inward positions. Therefore, because the detected positions of the address information differ for the case of a groove and for the case of a land, the land and groove can be differentiated from each other based on this difference in relative position.
FIG. 7 also shows an arrangement of four-segment photodetectors for converting return light into an electrical signal. The four-segment photodetectors comprises four detectors, A, B, C, and D and is a photodetector split into two components in the radial direction, into (A+D) and (B+C). In the on-track state on the land, the (A+D) photodetector detects address information ID1 and ID2 and the (B+C) photodetector detects address information ID3 and ID4. On the other hand, in the on-track state on the groove, the (A+D) photodetector detects address information ID3 and ID4 and the (B+C) photodetector detects address information ID1 and ID2.
FIG. 8 shows a signal representing (A+B+C+D) which is the total sum of the signals from the components A, B, C, and D of the four-segment photodetectors, that is, a signal representing the sum of two signals from the (A+D) photodetector and from (B+C) photodetector which are split into two in the radial direction. Because the laser spot 100 detects the address information ID1, ID2, ID3, and ID4 in that order, in the summation signal, a signal representing ID1 (ID1 signal), a signal representing ID2 (ID2 signal), a signal representing ID3 (ID3 signal), and a signal representing ID4 (ID4 signal) appear in that order. Because this signal is a summation signal, the polarity of all of the ID1, ID2, ID3, and ID4 signals are identical.
On the other hand, FIGS. 9 and 10 respectively show a difference signal between the (A+D) signal and (B+C) signal from the two photodetectors split into two in the radial direction, that is, a signal representing (A+D)−(B+C). FIG. 9 shows a difference signal in a groove sector. In a groove, the (B+C) detector detects the address information ID1 and ID2 and the (A+D) detector detects the address information ID3 and ID4, and thus, in the difference signal, the polarity of the ID1 and ID2 signals detected by the (B+C) detector is inverted.
FIG. 10 shows a waveform of a difference signal in a land sector. In a land, the (A+D) detector detects the address information ID1 and ID2 and the (B+C) detector detects the address information ID3 and ID4, and thus, in the difference signal, the polarity of the ID3 and ID4 signals is inverted.
In this manner, the address information ID1–ID4 can be obtained using either a summation signal or a difference signal, and thus, the address information of a DVD-RAM can be replayed. When the data section is to be replayed using a summation signal, the polarity of the signals representing the address information must be matched with the polarity of the summation signal. Therefore, when the address information is reproduced using a difference signal, the address information must be replayed with the polarity of ID1 and ID2 signals inverted on the groove sector and, similarly, on the land sector, the address information must be replayed with the polarity of ID3 and ID4 signals inverted. On the other hand, when the address information is reproduced using the summation signal, the polarity inverting process is not necessary.
As described, because the address information can be reproduced using either the summation signal or the difference signal of signals from two photodetectors, of the four-segment photodetectors, split into two in the radial direction, in conventional drives, the signal to be used is fixed to either the summation signal or the difference signal in the processing circuits for reproducing the address information. However, the present applicants have found that in some cases the address information cannot be reliably reproduced by fixing the signal to be used in this manner.
For example, in optical disk devices, there may be cases where a focus deviation occurs due to variation in precision during assembly of the optical pickup or the drive, insufficient adjustment of the focus offset, etc. When such focus deviation occurs, the shape of the laser light spot to be irradiated onto the optical disk is changed and the balance among the amounts of light from the four-segment photodetectorss A–D is disrupted. Because of this, even when the address information is to be reproduced using the difference signal, the in-phase noise cannot be removed, and in addition, an address reproduction error is generated by a phase change due to the difference calculation.
As shown in FIG. 11A, when the optical axis of the laser light irradiated from an optical pickup 14 is inclined with respect to the optical disk 10 (“tilt”), in addition to a main spot 100 which is the actual spot, a side lobe 200 appears near the main spot 100 as shown in FIG. 11B. In this case, as shown in FIG. 11C, a peak P2 appears in addition to the actual peak P1 in the distribution of the amount of light detected by the four-segment photodetectors. When address information is reproduced using a summation signal in this case, the resolution may be degraded or the phase may be changed because of the influence from the side lobe 200, and address reproduction error may occur similar to the above. In other words, regardless of whether a summation signal or a difference signal is used, an address reproduction error may be generated as long as the type (summation or difference) of the used signal is fixed.